Most of the power and usefulness of today's digital integrated circuit (IC) devices can be attributed to the increasing levels of integration. More and more components (resistors, diodes, transistors, and the like) are continually being integrated into the underlying chip, or IC. The starting material for typical ICs is very high purity silicon. The material is grown as a single crystal. It takes the shape of a solid cylinder. This crystal is then sawed (like a slice of bread) to produce wafers typically 10 to 30 cm in diameter and 250 microns thick.
The geometry of the features of the IC components is commonly defined photographically through a process known as photolithography. Very fine surface geometry can be reproduced accurately by this technique. The photolithography process is used to define component regions and build up components one layer on top of another. Complex ICs can often have many different built up layers, each layer having components, each layer having differing interconnections, and each layer stacked on top of the previous layer. The resulting topography of these complex IC's often resemble familiar terrestrial "mountain ranges," with many "hills" and "valleys" as the IC components are built up on the underlying surface of the silicon wafer.
In the photolithography process, a mask image, or pattern, defining the various components, is focused onto a photosensitive layer using incident light. The image is focused onto the surface using the optical means of the photolithography tool, and is imprinted into the photosensitive layer. To build ever-smaller features, increasingly fine images must be focused onto the surface of the photosensitive layer, i.e. optical resolution must increase. As optical resolution increases, the depth of focus of the mask image correspondingly narrows. This is due to the narrow range in depth of focus imposed by the high numerical aperture lenses in the photolithography tool. This narrowing depth of focus is often the limiting factor in the degree of resolution obtainable, and thus, the smallest components obtainable using the photolithography tool. The extreme topography of complex ICs, the "hills" and "valleys," exaggerate the effects of decreasing depth of focus. Thus, in order to properly focus the mask image defining sub-micron geometries onto the photosensitive layer, a precisely flat surface is desired. The precisely flat (i.e., fully planarized) surface will allow for extremely small depths of focus, and in turn, allow the definition and subsequent fabrication of extremely small components.
Chemical mechanical polishing (CMP) is a preferred method of obtaining full planarization of a semiconductor wafer. It involves removing a sacrificial layer of dielectric material using mechanical contact between the wafer and a moving polishing pad saturated with slurry. Polishing flattens out height differences, since high areas of topography (hills) are removed faster than areas of low topography (valleys). Polishing is the only technique with the capability of smoothing out topography over millimeter scale planarization distances leading to maximum angles of much less than one degree after polishing.
FIG. 1 is a top view of a chemical mechanical polishing (CMP) machine 100 and FIG. 2 is a side view of CMP machine 100. CMP machine 100 is fed semiconductor wafers to be polished. CMP machine 100 picks up the wafers with an arm 101 and places them onto a rotating polishing pad 102. Polishing pad 102 is made of a resilient material and is textured, often with a plurality of predetermined grooves 103, to aid the polishing process. Polishing pad 102 rotates on a platen 104, or turntable located beneath polishing pad 102, at a predetermined speed. A wafer 105 is held in place on polishing pad 102 within a carrier ring 112 that is connected to a carrier film 106 of arm 101. The front surface of wafer 105 rests against polishing pad 102. The back surface of wafer 105 is against the lower surface of carrier film 106 of arm 101. As polishing pad 102 rotates, arm 101 rotates wafer 105 at a predetermined rate. Arm 101 forces wafer 105 into polishing pad 102 with a predetermined amount of down force. CMP machine 100 also includes a slurry dispense arm 107 extending across the radius of polishing pad 102, which dispenses a flow of slurry onto polishing pad 102.
The slurry is a mixture of deionized water and polishing agents designed to chemically aid the smooth and predictable planarization of wafer 105. The rotating action of both polishing pad 102 and wafer 105, in conjunction with the polishing action of the slurry, combine to planarize, or polish, wafer 105 at some nominal rate. This rate is referred to as the removal rate. A constant and predictable removal rate is important to the uniformity and throughput performance of the wafer fabrication process. The removal rate should be expedient, yet yield precisely planarized wafers, free from surface anomalies. If the removal rate is too slow, the number of planarized wafers produced in a given period of time decreases, hurting wafer throughput of the fabrication process. If the removal rate is too fast, the CMP planarization process will not be consistent across several wafers in a batch, thereby hurting the consistency of the fabrication process.
To aid in maintaining a stable removal rate, CMP machine 100 includes a conditioner assembly 120. Conditioner assembly 120 includes a conditioner arm 108, which extends across the radius of polishing pad 102. An end effector 109 is connected to conditioner arm 108. End effector 109 includes an abrasive conditioning disk 110 that is used to roughen the surface of polishing pad 102. Conditioning disk 110 is rotated by conditioner arm 108 and is transitionally moved towards the center of the polishing pad 102 and away from the center of polishing pad 102, such that conditioning disk 110 covers the radius of polishing pad 102. In so doing, conditioning disk 110 covers the surface area of polishing pad 102, as polishing pad 102 rotates. A polishing pad having a roughened surface has an increased number of micro-pits and gouges in its surface from conditioner assembly 120 and therefore produces a faster removal rate via increased slurry transfer to the surface of wafer 105. Without conditioning, the surface of polishing pad 102 is smoothed during the polishing process and removal rate decreases dramatically. Conditioner assembly 120 re-roughens the surface of polishing pad 102, thereby improving the transport of slurry and improving the removal rate.
As described above, the CMP process uses abrasive slurry on a polishing pad. The polishing action of the slurry is comprised of an abrasive frictional component and a chemical component. The abrasive frictional component is due to the friction between the surface of the polishing pad, the surface of the wafer, and the abrasive particles suspended in the slurry. The chemical component is due to the presence in the slurry of polishing agents that chemically interact with the material of the dielectric layer of wafer 105. The chemical component of the slurry is used to soften the surface of the dielectric layer to be polished, while the frictional component removes material from the surface of wafer 105.
Referring still to FIGS. 1 and 2, the polishing action of the slurry determines the removal rate and removal rate uniformity, and thus, the effectiveness of the CMP process. As slurry is "consumed" in the polishing process, the transport of fresh slurry to the surface of wafer 105 and the removal of polishing by-products away from the surface of wafer 105 becomes very important in maintaining the removal rate. Slurry transport is facilitated by the texture of the surface of polishing pad 102. This texture is comprised of both predefined grooves 103 and micro-pits that are manufactured into the surface of polishing pad 102 and the inherently rough surface of the material from which polishing pad 102 is made.
The slurry is transported by grooves 103 and micro-pits of polishing pad 102 under the edges of wafer 105 as both polishing pad 102 and wafer 105 rotate. Consumed slurry and polishing by-products, in a similar manner, are also transported by grooves 103 and micro-pits of polishing pad 102 away from the surface of wafer 105. As the polishing process continues, fresh slurry is continually dispensed onto polishing pad 102 from slurry dispense arm 107. The polishing process continues until wafer 105 is sufficiently planarized and removed from polishing pad 102.
It should be appreciated that the CMP process described above is used as part of the fabrication of metal lines within a semiconductor wafer. For instance, to couple the various discrete components of a circuit, a conductor pattern is constructed between the components formed on the wafer. The conductor pattern is formed in a manner similar to that used to form the semiconductor devices. Oxidation is used to create a dielectric layer to isolate the conductor from the semiconductor portion of the wafer. Etching is used to define trenches for conductors. Chemical or physical vapor deposition is used to deposit a metal (e.g., copper) layer on the dielectric layer. Finally, chemical mechanical polishing (CMP) is typically used to remove the layer of metal from specific areas, usually the non-trench areas of the wafer that are not designed to be conductors. However, after the polishing operation, metal still remains within the trenches. The resultant product is a semiconductor wafer with metal-filled trenches that couple components. In this way, current is only conducted through the conducting metal line in the trenches between desired components in an IC circuit. The dielectric layer surrounding the trench prevents current leakage and short-circuits.
The following figures present the conventional prior art process for fabricating copper lines within a semiconductor wafer. As mentioned above, a pattern of trenches is defined within a dielectric layer as part of the prior art process of forming copper lines to electrically couple various discrete components. Prior art FIG. 3A is a top view of a semiconductor wafer 300 having a trench 304 defined within a dielectric layer 310 to eventually couple electronic components 307 and 308 electrically. A typical integrated circuit (IC) is made up of many electronic components that have many trenches formed between them. However, for simplicity, only trench 304 and electronic components 307 and 308 are shown.
Prior art FIG. 3B is a side sectional view of semiconductor wafer 300, along line 1--1 of FIG. 3A, after a layer of copper 302 is deposited above dielectric layer 310. It should be appreciated that dielectric layer 310 is deposited above a semiconductor substrate 301. Furthermore, dielectric layer 310 has a trench 304 formed therein. As such, copper layer 302 fills in trench 304 as it covers dielectric layer 310. Typically, a copper chemical mechanical polishing process is then used to remove copper layer 302 from areas 312 and 313 of semiconductor wafer 300. Furthermore, the copper CMP process is also used to planarize the upper surface of copper layer 302 remaining within trench 304. Following the copper CMP process, copper layer 302 ideally remains only within trench 304.
Prior art FIG. 3C illustrates semiconductor wafer 300 following the CMP process to remove copper layer 302 from areas 312 and 313, thereby leaving a copper line 302a within trench 304. Unfortunately, there are disadvantages associated with the prior art copper line fabrication process described above. One of the main disadvantages is that it typically results in a dished, or concave, surface 309, of copper line 302a. Specifically, the copper CMP process is implemented to expressly remove material from copper layer 302. As such, the copper CMP process removes material from copper layer 302 faster than it removes material from dielectric layer 310, resulting in dished surface 309. It should be appreciated that the dishing of surface 309 increases as the width 316 of trench 304 increases.
The resultant dished surface 309 of copper line 302a yields a variable resistance within copper line 302a. Specifically, variations of the cross-sectional area of copper line 302a between electronic components 307 and 308 of FIG. 3A create a variable metal (sheet) resistance within copper line 302a. The variable resistance affects the timing and the voltage level of the signals transferred between electronic components 107 and 108. Variation in the timing and the voltage of the signals transferred between components 107 and 108 have a subsequent effect on the overall IC performance. Timing and voltage levels affect interaction between parallel and serial circuitry in the IC and their respective logic levels. Consequently, a need exists for a method and system for providing copper lines within a semiconductor wafer that do not have dished surfaces.
While it is not shown in Prior Art FIG. 3A, 3B, or 3C, it is appreciated that within the prior art, a barrier (seed) layer, of material is deposited on the surfaces that form trench 304 prior to depositing copper layer 302. The barrier layer prevents copper layer 302 from diffusing into dielectric layer 310. The same or different barrier layer is also typically deposited on top of copper line 302a to prevent copper line 302a from diffusing into subsequent layers deposited on top of it. Within the prior art, a deposition of a barrier layer on top of copper line 302a is a discrete operation following the copper CMP process described above. As such, a specific amount of fabrication steps are utilized to fabricate and encapsulate copper line 302a within barrier layers. Hence, a need exists for a method and system that does not require additional fabrication steps to provide copper lines within a semiconductor wafer that do not have dished surfaces and are also encapsulated within barrier layers.
Thus, a need exists for a method and system for providing copper lines within a semiconductor wafer that do not have dished surfaces. A further need exists for a method and system which meets the above need but which does not require additional fabrication steps to encapsulate the copper lines of the semiconductor wafer within barrier layers.